Fail safe gas furnace optical flame sensor using a transconductance amplifier and low photodiode current

Abstract

A fail safe gas furnace optical flame sensor uses a transconductance amplifier with low photodiode current to sense the presence or absence of a gas flame within the burner of a gas furnace. The photodiode signal appears as the only negative voltage signal in the circuit, and the equivalent resistance feedback network is redundantly designed, thus ensuring that no false flame-on conditions will be detected due to the failure of a single resistive component. Because it does not reside within the flame, the sensor is immune to false flame-off conditions caused by material deposition and corrosion of the sensor.

Therefore, the following is claimed: 
     
       1. An optical flame sensor, comprising: a photodiode flame sensor operating in a photovoltaic short circuit mode designed to produce a low output current electrical signal when a flame is detected;   a transconductance amplifier operating in said photovoltaic short circuit mode connected via a transistor to said flame sensor, said transconductance amplifier including a feedback network designed to conduct said low output current from said flame sensor causing said transconductance amplifier to output a voltage high signal;   voltage comparator circuitry designed to compare said transconductance amplifier voltage high signal with a threshold voltage signal in order to develop a logic level output signal for input to a processor, and   test circuitry designed to provide a test signal, said test signal designed to interrupt said low output current from said flame sensor in order to interrogate the functionality of said flame sensor during a test pulse.   
     
     
       2. The flame sensor according to claim 1, wherein said processor contains logic designed to determine whether said flame sensor is detecting a flame. 
     
     
       3. The flame sensor according to claim 1, wherein said test signal is applied during an operating condition when said voltage high signal is present, said test signal causing said voltage high signal to be switched low during a test pulse. 
     
     
       4. The flame sensor according to claim 1, wherein said test signal is applied during a no light test condition when said voltage high signal is absent, said test signal causing the capacitive coupling of negative test pulse current at said transconductance amplifier input, causing said transconductance amplifier output to go into a high state, allowing the testing of said flame sensor in the absence of a flame. 
     
     
       5. The flame sensor according to claim 1, wherein said transconductance amplifier feedback network comprises a redundant tee circuit that allows a large equivalent impedance while using low value resistive components. 
     
     
       6. A method for detecting the presence of a gas flame, comprising the steps of: operating a photodiode flame sensor in a photovoltaic short circuit mode in order to produce a low output current electrical signal when a flame is detected;   operating a transconductance amplifier that is connected via a transistor to said flame sensor in said photovoltaic short circuit mode, said transconductance amplifier designed to output a voltage high signal when a flame is detected by said photodiode flame sensor;   operating a voltage comparator circuit designed to compare said transconductance amplifier voltage high signal with a threshold voltage signal in order to develop a logic level output signal for input to a processor, and   sending a test pulse, said test pulse designed to interrupt said low output current from said flame sensor in order to interrogate the functionality of said flame sensor during a test pulse.   
     
     
       7. The method according to claim 6, wherein said microprocessor contains logic designed to determine whether said flame sensor is detecting a flame. 
     
     
       8. The method according to claim 6, wherein said test signal is applied during an operating condition when said voltage high signal is present, said test signal causing said voltage high signal to be switched low during a test pulse. 
     
     
       9. The method according to claim 6, wherein said test signal is applied during a no light test condition when said voltage high signal is absent, said test signal causing the capacitive coupling of negative test pulse current at said transconductance amplifier input, causing said transconductance amplifier output to go into a high state, allowing the testing of said flame sensor in the absence of a flame. 
     
     
       10. The method according to claim 6, wherein said transconductance amplifier feedback network comprises a redundant tee circuit that allows a large equivalent impedance while using low value resistive components.